Attic floor area
Use the attic floor footprint – usually the same as the ceiling area of the top floor below. For a simple home, this equals the top floor square footage.
Ventilation ratio
1:300 is allowed when you have a Class I or II vapor retarder on the warm side, or when at least 40% but not more than 50% of vents are in the upper portion of the attic.
Vent configuration
Balanced means soffit (intake) and ridge or gable (exhaust) split roughly 50/50. This is required for the 1:300 ratio and is the IRC-preferred configuration.
Vent types you plan to use
NFVA = Net Free Vent Area is the actual open area after screens and louvers. It is printed on every code-compliant vent.
Ventilation required
Total NFVA needed
0
sq inches total
Intake (soffit)
0
sq inches
Exhaust (ridge/gable)
0
sq inches
Total in sq ft
0
overall NFVA
Attic ventilation prevents ice dams in winter, heat buildup in summer, and moisture damage year-round. IRC R806 requires 1 sq ft of net free vent area per 150 sq ft of attic floor, reducible to 1:300 with balanced vents or a vapor retarder. Never mix powered attic fans with ridge or gable vents – the fan will short-circuit and pull conditioned air from the house.